Protected retaining bands

ABSTRACT

An improved downhole tool apparatus for limiting the extrusion of a sealing elements in downhole tools that use segmented retaining assemblies, retaining shoes or retaining limiters. The apparatus provides for locating the retaining bands for the retaining assemblies in a groove on the inner surface of the retaining assembly so that the bands are protected from breaking prematurely by inadvertently contacting the wellbore, casing within a wellbore, or other object.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part of U.S. applicationSer. No. 13/448,060 filed Apr. 16, 2012 now abandoned, and claims thebenefit thereof.

BACKGROUND

The present invention relates to packer, bridge plug and frac plugs typetools used in wellbores and more particularly to retaining assemblies,such as extrusion limiters or retaining shoes, used in packer and bridgeplug type tools.

In the drilling or reworking of oil wells, a great variety of downholetools are used. For example, but not by way of limitation, it is oftendesirable to seal tubing or other pipe in the casing of the well, suchas when it is desired to pump cement or other slurry down the tubing andforce the cement or slurry around the annulus of the tubing or out intoa formation. It then becomes necessary to seal the tubing with respectto the well casing and to prevent the fluid pressure of the slurry fromlifting the tubing out of the well or for otherwise isolating specificzones in a well. Downhole tools referred to as packers and bridge plugsare designed for these general purposes and are well known in the art ofproducing oil and gas.

When it is desired to remove many of these downhole tools from awellbore, it is frequently simpler and less expensive to mill or drillthem out rather than to implement a complex retrieving operation. Inmilling, a milling cutter is used to grind the packer or plug, forexample, or at least the outer components thereof, out of the wellbore.In drilling, a drill bit is used to cut and grind up the components ofthe downhole tool to remove it from the wellbore. This is a much fasteroperation than milling, but requires the tool to be made out ofmaterials that can be accommodated by the drill bit. To facilitateremoval of packer type tools by milling or drilling, packers and bridgeplugs have been made, to the extent practical, of non-metallic materialssuch as engineering grade plastics and composites.

Packer tools and other wellbore isolation devices sometimes haveelements that undesirably protrude radially and inadvertently contact awellbore, a casing within a wellbore, or other object. Such contactsometimes results in damage to the packer tool and/or prematuretransitioning of the device from a run in configuration to a setconfiguration. For example, some conventional slip segments of wellboreisolation devices are held together somewhat tightly against a mandrelthrough the use of one or more bands. The bands may be intended tostretch or fracture when the tool is activated in order to allowdeployment. However, the bands often protrude radially and, thus, offerlimited resistance to inadvertent deployment when the wellbore isolationdevice undergoes inadvertent perturbation.

SUMMARY

The present invention provides a downhole apparatus that is moreresistant to inadvertent deployment than prior art downhole apparatuses.

In one embodiment of the invention there is provided a downholeapparatus for use in a wellbore. The apparatus has a mandrel having alongitudinal axial centerline and a radial direction perpendicular tothe longitudinal axial centerline. A sealing assembly is disposed aboutthe mandrel. The sealing assembly is radially expandable from an unsetposition to a set position in response to the application of axial forceon the sealing assembly. In the set position the sealing assemblyengages the wellbore. The invention also includes at least one retainingassembly for retaining the sealing assembly and resisting extrusion ofthe sealing assembly. The retaining assembly is proximate to the sealingassembly and has a plurality of segments disposed about the mandrel. Theplurality of segments is adapted to resist extrusion of the sealingassembly and adapted to expand radially to engage the wellbore when thesealing assembly is in the set position. When the sealing assembly is inthe unset position, the segments define an outer surface facing thewellbore and at least one end surface extending from the outer surfacetowards the mandrel. The end surface has a groove that extends aroundthe end surface wherein the groove is not exposed to the wellbore.Additionally, at least when the sealing assembly is in the unsetposition, the retaining assembly further comprises a band positioned inthe groove and suitable for holding the plurality of segments in placeabout the mandrel.

In another embodiment of the invention there is provided a retainingassembly for limiting the extrusion of a sealing assembly disposed abouta mandrel. The sealing assembly is movable from an unset position to aset position in a wellbore, and the sealing assembly seals the wellborewhen moved to the set position. The retaining assembly has a pluralityof segments with each segment adjacent to at least one other segment.When the sealing assembly is in the unset position the segments define:an inner surface for encircling the mandrel; an outer surface; a firstend surface for engaging an end of the sealing assembly and wherein thefirst end surface extends from the inner surface to the outer surface;and a second end surface opposing the first end surface and extendingfrom the inner surface to the outer surface. Additionally, a firstgroove extends around the first end surface and a second groove extendsaround the second end surface. The first and second grooves are spacedfrom said outer surface. When in place about the mandrel and when thesealing assembly is in the unset position the retaining assembly furtherhas a first band positioned in the first groove and a second bandpositioned in the second groove. The first band and second band aresuitable for holding the plurality of segments in place about themandrel while the sealing assembly is in the unset position.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side view of a downhole apparatus havingretaining assemblies embodying the present invention.

FIG. 2A is a cross-sectional side view of a sealing assembly andretaining assemblies of the embodiment of FIG. 1. of the presentinvention.

FIG. 2B is a cross-sectional side view of a sealing assembly andretaining assemblies of another embodiment of the present invention.

FIG. 3 is a cross-sectional side view of the downhole apparatus of theembodiment of FIG. 1 in a set position.

FIG. 4 is a front view of a retaining assembly of the present invention.

FIG. 5 is a perspective view of a single retaining assembly segment.

FIG. 6 is a perspective view of the retaining assembly of the presentinvention.

FIG. 7 is a perspective view of the retaining assembly of the presentinvention as viewed from the opposite side as the view of FIG. 6.

FIG. 8 is a cross-sectional side view of a prior art packer element andretaining assembly,

DETAILED DESCRIPTION

Referring now to FIGS. 1, 2 and 3, downhole tool, or downhole apparatus10 is shown in an unset position 11 (FIGS. 1 and 2) or a set position 13(FIG. 3) in a well 15 having a wellbore 20. The wellbore 20 can beeither a cased completion with a casing 22 cemented therein as shown inFIG. 1 or an openhole completion. Generally, as used here in the term“wellbore” will refer to either a cased completion or an openholecompletion. Downhole apparatus 10 is shown in set position 13 in FIG. 3.Casing 22 has an inner surface 24. An annulus 26 is defined by casing 22and downhole tool 10. Downhole tool 10 has a mandrel 28, and may bereferred to as a bridge plug due to the downhole tool 10 having a plug30 being pinned within mandrel 28 by radially oriented pins 32. Plug 30has a seal means 34 located between plug 30 and the internal diameter ofmandrel 28 to prevent fluid flow therebetween. The overall downhole tool10 structure, however, is adaptable to tools referred to as packers, andfrac plugs which typically have at least one means for allowing fluidcommunication through the tool. Packers may therefore allow for thecontrolling of fluid passage through the tool by way of one or morevalve mechanisms which may be integral to the packer body or which maybe externally attached to the packer body. Frac plugs control fluidpassage through the use of a frac ball. Such valve mechanisms are notshown in the drawings of the present document. Packer tools may bedeployed in wellbores having casings or other such annular structure orgeometry in which the tool may be set.

Mandrel 28 has an outer surface 36, an inner surface 38, and alongitudinal central axis, or longitudinal axial centerline 40. Also, asreferred to herein the term “radially” will refer to a radial directionperpendicular to the longitudinal axial centerline. An inner tube 42 isdisposed in, and is pinned to, mandrel 28 to help support plug 30.

Downhole tool 10, which as illustrated is a packer apparatus, includesthe usage of a spacer ring 44 which is preferably secured to mandrel 28by pins 46. Spacer ring 44 provides an abutment, which serves to axiallyretain slip segments 48 which are positioned circumferentially aboutmandrel 28. Slip retaining bands 50 serve to radially retain slipsegments 48 in an initial circumferential position about mandrel 28 aswell as slip wedge 52. Bands 50 are made of a steel wire, a plasticmaterial, or a composite material having the requisite characteristicsof having sufficient strength to hold the slip segments 48 in placeprior to actually setting the downhole tool 10 and to be easilydrillable when the downhole tool 10 is to be removed from the wellbore20. Preferably, bands 50 are inexpensive and easily installed about slipsegments 48. Slip wedge 52 is initially positioned in a slidablerelationship to, and partially underneath, slip segments 48 as shown inFIG. 1. Slip wedge 52 is shown pinned into place by pins 54. Designs ofslip segments 48 and co-acting slip wedges 52 are described in U.S. Pat.No. 5,540,279, which is incorporated herein by reference.

Located below slip wedge 52 is a sealing assembly 56, which includes atleast one sealing element, and as shown in FIG. 1 includes threeexpandable sealing elements 58 positioned about mandrel 28. In packertype tools such sealing elements are often referred to as packerelements. Sealing assembly 56 has upper end 60 and lower end 62. Sealingassembly 56 has unset and set positions 57 (FIG. 1) and 59 (FIG. 3)corresponding to the unset and set positions 11 and 13, respectively, ofdownhole tool 10. The sealing assembly 56 is radially expandable fromthe unset position 57 to a set position 59 in response to theapplication of axial force on the sealing assembly 56. In the setposition 59, the sealing assembly 56 engages the casing 22 to create aseal to prevent flow through annulus 26.

The present invention has retaining assemblies 66 disposed at the upperand lower ends 60 and 62 of sealing assembly 56 to axially retain thesealing assembly 56. Retaining assemblies 66 (also referred to asretaining shoes or extrusion limiters) may be referred to as an upperretaining assembly 68 and a lower retaining assembly 70. A slip wedge 72is disposed on mandrel 28 below lower retaining assembly 70 and ispinned with a pin 74. Located below slip wedge 72 are slip segments 76.Slip wedge 72 and slip segments 76 are like slip wedge 52 and slipsegments 48. At the lowermost portion of downhole tool 10 is an angledportion, referred to as mule shoe 78, secured to mandrel 28 by pin 79.The lowermost portion of downhole tool 10 need not be mule shoe 78 butcan be any type of section which will serve to terminate the structureof the downhole tool 10 or serve to connect the downhole tool 10 withother tools, a valve or tubing, etc. It will be appreciated by those inthe art that pins 32, 46, 54, 74, and 79, if used at all, arepreselected to have shear strengths that allow for the downhole tool 10to be set and deployed and to withstand the forces expected to beencountered in the wellbore 20 during the operation of the downhole tool10.

FIG. 8 shows a prior art arrangement of a retaining assemblies 150,which may referred to as retaining shoes or extrusion limiters. Upperand lower retaining assembly 152 and 154 are essentially identical.Therefore, the same designating numerals will be used to furtheridentify features on each of retaining shoes 152 and 154, which arereferred to collectively herein as retaining assemblies 150. Retainingassemblies 150 comprise an inner shoe, or inner retainer 156 and anouter shoe, or outer retainer 158 Inner and outer shoes 156 and 158 areheld in place by retaining bands 160, which are received in a groove162. Retaining bands 160 are exposed so that they can undergoinadvertent contact with a wellbore, a casing within a wellbore, orother object.

Referring now to FIGS. 2A and 4-7, the retaining assemblies 66 (alsocalled retaining shoes or extrusion limiters) of the present inventionwill be described. Upper and lower retaining assemblies 68 and 70 areessentially identical. Therefore, the same designating numerals will beused to further identify features on each of retaining assemblies 68 and70, which are referred to collectively herein as retaining assemblies66. Retaining assemblies 66 are preferably comprised of a plurality ofretainer segments, or shoe segments, 80 to form retaining assemblies 66that encircles mandrel 28. Retainer segments 80 can be made form anysuitable material that will withstand the downhole use and yet can bereadily cut or ground up by drilling with a drill bit. Generally,non-metallic engineering grade plastics can be used for the retainingmaterials, such as composite materials or structural phenolic materials.A suitable phenolic materials are available from General Plastics &Rubber Company, Inc., 5727 Ledbetter, Houston, Tex. 77087-4095.Alternatively, structural phenolics available from commercial suppliersmay be used. A suitable composite materials are available from GeneralPlastics & Rubber Company, Inc., 5727 Ledbetter, Houston, Tex.77087-4095. Particularly suitable materials for at least a portionretaining assemblies 66 includes direction specific composite materialavailable from General Plastics & Rubber Company, Inc.

Retaining assemblies 66 have an outer surface 82. Retaining assemblies66 also have an inner surface 84 composed of inner surface 86, first endsurface 88 and second end surface 90. When the downhole tool is in theunset position 11, retaining segments 80 define outer surface 82 andinner surface 84. Generally outer surface 82 will be substantiallycylindrical and face the wellbore 20. In the set position 13, the arcsurfaces 83 of retaining segments 66 making up outer surface 82 engagethe wellbore. Generally inner surface 86 will be a substantiallycylindrical inner surface, which encircles the mandrel. Timer surface 86is defined by arc surfaces 87 of retaining segments 66. Arc surfaces 87engage mandrel 28 in an initial or running position of the downhole tool10. First end surface 88 extends from the outer surface 82 to innercircle 86. Additionally, first end surface 88 extends in a generallycircumferential direction but is preferably not parallel to the radialdirection. As can best be seen from FIG. 6, first end surface 88 canhave an arcuate shaped cross-section or can be sloped. In the embodimentshown, first end surface 88 is shaped to accommodate the upper and lowerends 60 and 62 of the sealing assembly 56 and, thus, is preferablysloped as well as arcuate to provide a generally truncated conicalsurface which transitions from having a greater radius proximate outersurface 82 to a smaller radius proximate substantially cylindrical innersurface 86. Second end surface 90 opposes first end surface 88 and,hence, extends from the outer surface 82 to inner circle 86.Additionally, second surface 90 extends in a generally circumferentialdirection. Second end surface 90 may be generally parallel to the radialdirection or may be at a slight angle, preferably less than 10° fromparallel to the radial direction. However, it is within the scope of theinvention for both end surfaces (first end surface 88 and second endsurface 90) to have other shapes as long as they generally extendcircumferentially and from outer surface 82 to inner surface 86.

As shown in FIG. 2, upper and lower ends 60 and 62 of sealing assembly56 reside directly against upper and lower retaining assemblies 68 and70. Retaining assemblies 66 are preferably comprised of a plurality ofretainer segments 80 that encircles mandrel 28. Each retainer segment 80has ends 92 and 94, which can be flat and convergent with respect to acenter reference point, which, if the retainer segments 80 are installedabout mandrel 28, will correspond to the longitudinal central axis 40 ofthe mandrel 28 as depicted in FIG. 1. Ends 92 and 94 need not be flatand can be of other topology. In a preferred embodiment end 92 has ashelf 96 and end 94 has a tongue portion 98, as can be seen in FIGS. 5and 6. Tongue portion 98 is adapted to be received onto shelf 96 sothat, in the unset position 11, retainer segments overlap and form asubstantially continuous ring. Further tongue portion 98 and shelf 96are adapted so that, when retaining assemblies 66 are expanded in theset position 13, the retaining segments still overlap and extrusion ofthe sealing elements 58 through the gaps 118 between retaining segmentsis blocked by the tongue and shelf arrangement.

FIG. 4-7 illustrate retaining assemblies 66 being made of a total ofeight retainer segments 80 to provide a 360 degrees annulus encirclingstructure to provide a maximum amount of end support for sealingelements 58 to be retained in the axial direction. A lesser or greateramount of retainer segments 80 can be used depending on the nominaldiameters of the mandrel 28, the sealing elements 58, and the wellbore20 or casing 22 in which the downhole tool 10 is to be deployed. Innerdiameter 122 generally approaches the inner diameter of the sealingassembly 56. As is apparent from the drawings, outer surface 82 facesoutwardly away from the downhole tool 10. The slope of first end surface88 is preferably approximately 45 degrees as shown in FIG. 2. However,the exact slope will be determined by the exterior configuration of theends of the sealing elements 58 that are to be positioned and eventuallyplaced in contact with retaining assemblies 66 and first end surface 88.Inner surface 86 of retaining assembly 66 can be slightly sloped,approximately 5 degrees if desired, but it is best determined by thesurface of the downhole tool 10 which it eventually abuts against whendownhole apparatus 10 is centered in the wellbore 20.

Each retainer segment 80 can have a lug (protruding member) 100extending out from second end 90. As can be seen from FIGS. 1 and 2, thelugs 100 of upper retaining assembly 68 contacts or abuts a slip wedge52 such that an upper gap 110 is created when the downhole tool is inthe unset position 11. As can be seen in FIG. 3, when the downhole toolis moved to the set position, tipper retaining assembly 68 expandsallowing slip wedge 52 to slide under the lugs 100 and fill gap 110 suchthat the end of the slip wedge that abutted the lugs is now between thelugs and the mandrel. Similarly, the lugs 100 of lower retainingassembly 70 contacts slip wedge 72 such that a lower gap 112 is createdwhen the downhole tool is in the unset position. Also, when the downholetool is moved to the set position, upper retaining assembly 70 expandsallowing slip wedge 72 to slide under the lugs 100 and fill gap 112.

An important aspect of the current invention is groove 114 and 116.Groove 114 extends circumferentially around the first end surface 88.Groove 116 extends circumferentially around the second end surface 90.Retaining band 115 is positioned in groove 114 and retaining band 117 ispositioned groove 116. Retaining bands 115 and 116 are received ingrooves 114 and 116 to initially hold the retainer segments 80 in placeprior to setting the downhole tool 10 into the set position 13. It is apreferred embodiment that the grooves 114 and 116 and retaining bands115 and 117 be located on inner surface 84 instead of outer surface 82.More preferably the grooves 114 and 116 and retaining bands 115 and 117should be located on first end surface 88 and second end surface 90. Thegrooves 114 and 116 should be spaced from outer surface 82, i.e., notexposed to the wellbore 20 or not facing the wellbore 20. Location ofthe bands and grooves in these positions prevent contacts that mightfracture or release the bands and result in premature expansion of theretainer segments 80. Retaining bands 115 and 117 may be made of anonmetallic material, such as composite materials available from GeneralPlastics & Rubber Company, Inc., 5727 Ledbetter, Houston, Tex.77087-4095. However, bands 114 and 116 may be alternatively made of ametallic material such as ANSI 1018 steel or any other material havingsufficient strength to support and retain the retaining assembly 66 inposition prior to actually setting the downhole tool 10. Furthermore,retaining bands 115 and 117 may have either elastic or non-elasticqualities depending on how much radial, and to some extent axial,movement of the retainer segments 80 can be tolerated prior to enduringthe deployment of the associated downhole tool 10 into the wellbore 20.

In unset position 57, retaining bands 115 and 117 serve to hold retainersegments 80 in place. Prior to the downhole tool 10 being set, retainingassemblies 66 engage mandrel 28 about the upper and lower ends 60 and 62of the sealing assembly 56. Lower retaining assembly 70 engages lowerend 62 of sealing assembly 56 and upper retaining assembly 68 engagesthe upper end 60 of sealing assembly 56 in the unset positions 11 and 57of downhole tool 10 and the sealing assembly 56, respectively. When thedownhole tool 10 has reached the desired location in the wellbore 20,setting tools as commonly known in the art will move the downhole tool10 and, thus, the sealing assembly 56, to their set positions 13 and 59,respectively, as shown in FIG. 3.

Gaps 118 have a width 120 that can be essentially zero when the retainersegments 80 are initially installed about mandrel 28, and before thedownhole tool 10 is moved from the unset position 11 to the set position13. However, a small gap, for example a gap of 0.06″ may be provided foron initial installation. The width 120 of gap 118 will increase fromthat which exists on initial installation, as the downhole tool 10 isset.

When the downhole tool 10 is moved to its set position 13, retainingbands 115 and 117 will break and retaining assembly 66 will moveradially outwardly so that arc surfaces 83 of each retainer segment 80will engage inner surface 24 of casing 22. The radial movement willcause width 120 of gaps 118 to increase. However, the tongue portion 98and shelf 96 of retainer segments 80 will still overlap and, thus,extrusion of sealing elements 58 through gaps 118 and past retainingassembly 66 will be prevented. Additionally, the slip wedges 52 and 72will move under lugs 100, as described above. Accordingly, slip wedges52 and 72 will prevent extrusion of sealing elements 58 betweenretaining assembly 66 and mandrel 28 as illustrated in FIG. 3.

As can be understood from the foregoing description, the extrusion ofsealing elements 58 is essentially eliminated, since arc surface 83engage the wellbore 20 and prevent extrusion on the wellbore side of thedownhole tool. Additionally, any material extruded through gaps 118 willbe blocked by the tongue and shelf arrangement of the retainer segments,extrusion between retainer segments 80 and mandrel 28 is blocked by theslip wedges 52 and 72. Retaining assemblies 66 are thus expandableretaining shoes that will prevent or at least limit the extrusion of thesealing elements 58 and be less subject to premature expansion.Retaining assembly 66 may also be referred to as an expandable retainer.The arrangement is particularly useful in high pressure, hightemperature wells, since there is no extrusion path available. It shouldbe understood, however, that the disclosed retaining assembly 66 may beused in connection with packer-type tools of lesser or greaterdiameters, differential pressure ratings, and operating temperatureratings than those set forth herein.

Turning now to FIG. 2B an alternative embodiment of the invention isshown. In FIG. 2B similar parts to those in FIG. 2A have been given thesame reference number. In the embodiment of FIG. 2B there is upperretaining assembly 132 and lower retaining assembly 134. Upper and lowerretaining assemblies 134 are essentially identical. Therefore the samedesignating numerals will be used to further identify features on eachof retaining assemblies 132 and 134, which are referred to collectivelyherein as retaining assemblies 130. Retaining assemblies 130 comprisingan inner ring 136 and an outer ring 142. Inner ring 136 can have anarcuate or an angular cross section and mates with outer ring 142, suchthat radial portion 138 is between outer ring 142 and either the upperor lower end of the sealing assembly 60 or 62 and such that thelongitudinal or axial portion 140 of inner ring 136 is between mandrel28 and outer ring 142. Additionally, inner ring 136 can be comprised ofa plurality of segments with each segment adjacent to at least one othersegment. Outer ring 142 is essentially identical to retaining assemblies66, except that it has a larger inner diameter 122 to accommodate innerring 136.

Although the disclosed invention has been shown and described in detailwith respect to a preferred embodiment, it will be understood by thoseskilled in the art that various changes in the form and detailed areamay be made without departing from the spirit and scope of thisinvention as claimed. Thus, the present invention is well adapted tocarry out the object and advantages mentioned as well as those which areinherent therein. While numerous changes may be made by those skilled inthe art, such changes are encompassed within the spirit of thisinvention as defined by the appended claims.

What is claimed is:
 1. An apparatus for use in a wellbore, comprising: amandrel having a longitudinal axial centerline and a radial directionperpendicular to said longitudinal axial centerline; a sealing assemblydisposed about said mandrel, wherein said sealing assembly is radiallyexpandable from an unset position to a set position in response toapplication of axial force on said sealing assembly wherein said sealingassembly engages said wellbore in said set position; and a retainingassembly for retaining said sealing assembly and resisting extrusion ofsaid sealing assembly, said retaining assembly proximate to said sealingassembly and comprising: a plurality of segments disposed about saidmandrel, adapted to resist extrusion of said sealing assembly, andadapted to expand radially to engage said wellbore when said sealingassembly is in said set position; wherein, when said sealing assembly isin said unset position, said segments define: an outer surface facingsaid wellbore; a first end surface extending from said outer surfacetoward said mandrel and having a first groove that extends around saidfirst end surface; and a second end surface extending from said outersurface toward said mandrel and having a second groove that extendsaround said second end surface, wherein said first and second groovesare not exposed to said wellbore; a slip wedge encircling and slidablealong a portion of said mandrel wherein a first end of said slip wedgeabuts said second end surface and said first end surface abuts a firstend of said sealing assembly, and wherein said second end surface has atleast one protruding member positioned so that when said sealingassembly is in said unset position said first end of said slip wedgeabuts said protruding member, and when said sealing assembly is in saidset position said first end of slip wedge is between said mandrel andsaid protruding member.
 2. The apparatus of claim 1 wherein said firstgroove has a first band positioned therein, said second groove has asecond band positioned therein and wherein said first band and saidsecond bands are suitable for holding said plurality of segments inplace about said mandrel while said sealing assembly is in said unsetposition.
 3. The apparatus of claim 1 wherein each of said segments areadjacent to at least one segment and said thus adjacent segmentscircumferentially overlap each other such that, when said sealingassembly is in said set position, extrusion of said sealing assemblybetween said adjacent segments is blocked.
 4. The apparatus of claim 3,wherein adjacent segments have adjacent segment-facing ends, which havea tongue-and-shelf interaction such that the tongue on one of theadjacent segment-facing ends is received onto the shelf on anothersegment-facing end such that the adjacent segments circumferentiallyoverlap such that extrusion of said sealing assembly between saidadjacent segments is blocked when said sealing is in said set position.5. An apparatus for use in a wellbore, comprising: a mandrel having alongitudinal axial centerline and a radial direction perpendicular tosaid longitudinal axial centerline; a sealing assembly disposed aboutsaid mandrel, wherein said sealing assembly is radially expandable froman unset position to a set position in response to application of axialforce on said sealing assembly wherein said sealing assembly engagessaid wellbore in said set position; and a retaining assembly forretaining said sealing assembly and resisting extrusion of said sealingassembly, said retaining assembly proximate to said sealing assembly andcomprising: a plurality of segments disposed about said mandrel, adaptedto resist extrusion of said sealing assembly, and adapted to expandradially to engage said wellbore when said sealing assembly is in saidset position; wherein, when said sealing assembly is in said unsetposition, said segments define an outer surface facing said wellbore andat least one end surface extending from said outer surface towards saidmandrel; and wherein said end surface has a groove that extends aroundsaid end surface, wherein said groove is not exposed to said wellbore;and when said sealing assembly is in said unset position, said segmentsdefine: a first end surface extending from said outer surface towardsaid mandrel and having a first groove that extends end around saidfirst end surface, wherein said first groove is not exposed to saidwellbore, and as second end surface extending from said outer surfacetoward said mandrel and having a second groove that extends around saidsecond end surface, wherein said second groove is not exposed to saidwellbore; wherein when said sealing assembly is in said unset position,said first groove has a first band positioned therein, said secondgroove has a second band positioned therein and said first and saidsecond band are suitable for holding said plurality of segments in placeabout said mandrel; and each of said segments are adjacent to at leastone other segment and said thus adjacent segments have adjacentsegment-facing ends, which have a tongue-and-shelf interaction such thatthe tongue on one of the adjacent segment-facing ends is received ontothe shelf on another segment-facing end such that the adjacent segmentscircumferentially overlap each other such that, when said sealingassembly is in said set position, extrusion of said sealing assemblybetween said adjacent segments is blocked.
 6. A retaining assembly forlimiting extrusion of a sealing assembly disposed about a mandrel,wherein said sealing assembly is movable from an unset position to a setposition in a wellbore, and said sealing assembly seals said wellborewhen moved to said set position, said retaining assembly comprising: aplurality of segments with each segment adjacent to at least one othersegment and said thus adjacent segments have adjacent segment-facingends, which have a tongue-and-shelf interaction such that the tongue onone of the adjacent segment-facing ends is received onto the shelf onanother segment-facing end such that the adjacent segmentscircumferentially overlap each other such that, when said sealingassembly is in said set position, extrusion of said sealing assemblybetween said adjacent segments is blocked, and wherein, when saidsealing assembly is in said unset position, said segments define: aninner surface for encircling said mandrel; an outer surface exposed tothe wellbore; a first end surface for engaging an end of said sealingassembly and wherein said first end surface extends from said innersurface to said outer surface; and a second end surface opposing saidfirst end surface and extending from said inner surface to said outersurface; wherein said plurality of segments have a first groove thatextends around said first end surface and a second groove that extendsaround said second end surface and wherein said first groove and secondgroove are spaced from said outer surface.
 7. The apparatus of claim 6wherein said retaining assembly further comprises a first bandpositioned in said first groove and a second band positioned in saidsecond groove wherein said first band and second band are suitable forholding said plurality of segments in place about said mandrel whilesaid sealing assembly is in said unset position.
 8. The apparatus ofclaim 6 wherein each of said adjacent segments circumferentially overlapeach other such that, when said sealing assembly is in said setposition, extrusion of said sealing assembly between said adjacentsegments is blocked.
 9. The apparatus of claim 6 wherein said second endsurface has at least one lug positioned between said substantiallycylindrical outer surface and said second groove.
 10. The apparatus ofclaim 9 wherein: when said sealing assembly is in said unset position,said retaining assembly further comprises a first band positioned insaid first groove and a second band positioned in said second groovewherein said first band and second band are suitable for holding saidplurality of segments in place about said mandrel while said sealingassembly is in said unset position; and said second end surface has atleast one lug positioned between said substantially cylindrical outersurface and said second groove.
 11. Apparatus for use in a wellbore,comprising: a mandrel having a longitudinal axial centerline and aradial direction perpendicular to said longitudinal axial centerline; asealing assembly disposed about said mandrel, wherein said sealingassembly is radially expandable from an unset position to a set positionin response to application of axial force on said sealing assemblywherein said sealing assembly engages said wellbore in said setposition, and a first retaining assembly for limiting extrusion of thesealing assembly, said retaining assembly comprising: a plurality ofsegments with each segment adjacent to at least one other segment andsaid thus adjacent segments have adjacent segment-facing ends, whichhave a tongue-and-shelf interaction such that the tongue on one of theadjacent segment-facing ends is received onto the shelf on anothersegment-facing end such that the adjacent segments circumferentiallyoverlap each other such that, when said sealing assembly is in said setposition, extrusion of said sealing assembly between said adjacentsegments is blocked, and, wherein when said sealing assembly is in saidunset position, said segments define: an inner surface for encirclingsaid mandrel; an outer surface; a first end surface for engaging an endof said sealing assembly and wherein said first end surface extends fromsaid inner surface to said outer surface; and a second end surfaceopposing said first end surface and extending from said inner surface tosaid outer surface; wherein said plurality of segments have a firstgroove that extends around said first end surface and a second groovethat extends around said second end surface, and wherein said firstgroove and said second groove are spaced from said outer surface. 12.The apparatus of claim 11 wherein said first retaining assembly furthercomprises a first band positioned in said first groove and a second bandpositioned in said second groove wherein said first band and second bandare suitable for holding said plurality of segments in place about saidmandrel while said sealing assembly is in said unset position.
 13. Theapparatus of claim 11 further comprising a slip wedge encircling andslidable along a portion of said mandrel wherein a first end of saidslip wedge abuts said second end surface.
 14. The apparatus of claim 13wherein said second end surface has at least one lug positioned so thatwhen said sealing assembly is in said unset position said first end ofsaid slip wedge abuts said lug, and when said sealing assembly is insaid set position said first end of slip wedge is between said mandreland said lug.
 15. The apparatus of claim 11 further comprising a slipwedge encircling and slidable along a portion of said mandrel andwherein: when said sealing assembly is in said unset position, saidfirst retaining assembly further comprises a first band positioned insaid first groove and a second band positioned in said second groovewherein said first band and second band are suitable for holding saidplurality of segments in place about said mandrel while said sealingassembly is in said unset position; each of said adjacent segmentscircumferentially overlap each other such that, when said sealingassembly is in said set position, extrusion of said sealing assemblybetween said adjacent segments is blocked; and said second end surfacehas at least one lug positioned so that when said sealing assembly is insaid unset position a first end of said slip wedge abuts said lug, andwhen said sealing assembly is in said set position said first end ofslip wedge is between said mandrel and said lug.